1. Field of the Invention
The present invention relates to an image reading device equipped with a linear sensor array.
2. Description of the Prior Art
Charge-coupled device (CCD) has been employed as a linear image sensor in the image reading device for use in a facsimile, a copier or the like. Such CCD image sensor is generally composed of a photoelectric converting section for example with plural photodiodes for generating electric charges in response to incident light, a transfer section for accumulating said charges and transferring them to an output section, and an output section provided with a transistor for converting thus transferred charges into voltage signals.
For reading a large original image with a high resolution power, there is required a CCD having a large number of cells and capable of high speed charge transfer. Consequently the transfer section of CCD is often divided into two channels respectively corresponding to even and odd photodiodes.
FIG. 1 shows such conventional linear image sensor comprising a photoelectric converting section PD having plural photodiodes; a charge transfer section CCD A corresponding to odd photodiodes; a charge transfer section CCD B corresponding to even photodiodes; and an output section 0, wherein a continuous video signal V of a scanning line is obtained by synthesizing signals transferred in two separate channels respectively through CCD A and CCD B into one channel in said output section 0.
Such linear image sensor having two CCD channels can achieve an improved efficiency, since each cell C in the CCD channels can be doubled in size and the number of cells in each channel as well as the signal transfer rate in each channel can be halved in comparison with the case of transferring all the pixels through a single CCD channel.
However, in the image reading device equipped with the CCD image sensor shown in FIG. 1, the signals generated by the odd photodiodes and those generated by the even photodiodes are transferred respectively in different channels CCD A and CCD B, so that there may result a difference between the levels of the video signal corresponding to the odd pixels and that corresponding to the even pixels, due to a difference between the characteristics of said CCD channels. Such a difference in the levels of the video signals V results in a deterioration of the reproduced image.
Also the image signal synthesized in the output section 0 requires the use of circuit components, such as amplifiers and A/D converters, of a large band width, and this fact inevitably raises the cost of the image reading device and limits the image reading speed thereof.